Boundary layer flow sensor

ABSTRACT

Apparatus and methods described herein provide for boundary layer flow sensor and corresponding determination of the flow characteristics of an ambient airflow over an aerodynamic surface. According to one aspect of the disclosure provided herein, the boundary layer flow sensor includes a body configured for mounting within or below the aerodynamic surface, a pressure port configurable between an open state for taking pressure measurements within the boundary layer of the ambient airflow and a closed state that protects the pressure port from contaminants when not in use.

BACKGROUND

Ambient airflow over an aerodynamic surface creates a boundary layer atthe surface over which it flows. The airflow at the aerodynamic surfacemay be thought of as having zero velocity at the precise locationabutting the surface due to the viscosity at the surface, speeding up tothe mean velocity of the ambient airflow at a distance from the surface.The airflow within this distance defines the boundary layer. The airflowwithin a boundary layer may be generally characterized as laminar orturbulent. Laminar flow is generally associated with lower skinfriction, lower flow velocity near the surface, and thinner boundarylayer thickness as compared with turbulent flow. As a result, laminarflow is often desirable in order to reduce aerodynamic drag.

Determining whether an airflow is laminar or turbulent may be done invarious ways, each of which is undesirable for various reasons.Conventional techniques include using an infrared camera to measuresurface temperature, using a hot film to measure sheer stress, or usinga pressure tube mounted on the surface to measure the total pressure ofthe airflow. These techniques are often cumbersome, not practical forroutine flight operations, not useful in all desired locations, andcreate additional drag while lacking the robustness needed for continueduse in actual flight operations.

It is with respect to these considerations and others that thedisclosure made herein is presented.

SUMMARY

It should be appreciated that this Summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary is not intended to beused to limit the scope of the claimed subject matter.

Apparatus and methods described herein provide for a boundary layer flowsensor and corresponding method for determining the characteristics ofan ambient airflow over an aerodynamic surface. According to one aspectof the disclosure provided herein, a boundary layer flow sensor includesa body, a pressure port, and an activation mechanism. The body isconfigured for mounting below an aerodynamic surface and has a pressuretube extending along a longitudinal axis of the body. The pressure portis configurable between an open state and a closed state. In the openstate, the pressure tube is fluidly coupled to a lower portion of aboundary layer of the ambient airflow over the aerodynamic surface. Inthe closed state, the pressure tube is decoupled from the boundarylayer.

According to another aspect, a boundary layer flow sensor includes abody configured for mounting within a fastener aperture within anaerodynamic surface, and an activation mechanism. The body includes amovable shaft that is moveable between a raised position to create anopen state for a pressure port and a lowered position to create a closedstate for the pressure port. In the closed state, the body issubstantially flush with the aerodynamic surface. The moveable shaftincludes a total pressure tube extending along a longitudinal axis ofthe movable shaft and is substantially aligned with the ambient airflowwhen the pressure port is in the open state. The moveable shaft alsoincludes at least one static pressure tube positioned at an offset anglefrom the total pressure tube. The activation mechanism is operative toraise the moveable shaft and fluidly couple the total pressure tube andthe static pressure tube to a lower portion of a boundary layer of theambient airflow, and to lower the movable shaft to the lowered positionsubstantially flush with the aerodynamic surface and fluidly decouplethe total pressure tube and the static pressure tube from the lowerportion of the boundary layer of the ambient airflow.

According to yet another aspect, a method for determining flowcharacteristics of an ambient airflow over an aerodynamic surfaceincludes activating a boundary layer flow sensor that is mountedsubstantially flush with the aerodynamic surface. In response toactivation, a pressure port is exposed to a lower portion of a boundarylayer of the ambient airflow. At least one pressure is measured withinthe ambient airflow and used to determine whether the ambient airflow issubstantially laminar or turbulent.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a boundary layer flow sensor in closed and openstates, the boundary layer flow sensor having a hinged cover accordingto one embodiment presented herein;

FIG. 2 is a side view of a boundary layer flow sensor in closed and openstates, the boundary layer flow sensor having a deformable coveraccording to one embodiment presented herein;

FIGS. 3A and 3B are top perspective and side views, respectively, of aboundary layer flow sensor in closed and open states, the boundary layerflow sensor having a movable shaft according to one embodiment presentedherein;

FIGS. 4A and 4B are top perspective and side views, respectively, of aboundary layer flow sensor in closed and open states, the boundary layerflow sensor having a movable shaft according to an alternate embodimentpresented herein;

FIGS. 5A and 5B are top perspective and side views, respectively, of aboundary layer flow sensor in closed and open states, the boundary layerflow sensor having a movable shaft with a lid according to oneembodiment presented herein;

FIG. 6 is cross-sectional view of a boundary layer flow sensor having amovable shaft with a lid, total pressure tube, and static pressure tubesaccording to one embodiment presented herein;

FIG. 7 is top perspective view of the boundary layer flow sensor of FIG.6 according to one embodiment presented herein;

FIG. 8 is a block diagram illustrating various embodiments of anactivation mechanism according to the disclosure provided herein;

FIG. 9 is a flow diagram illustrating a method for determining flowcharacteristics of an ambient airflow over an aerodynamic surface; and

FIG. 10 is a graph illustrating a pressure parameter versus distancealong a chord for both laminar and turbulent flows according to oneembodiment presented herein.

DETAILED DESCRIPTION

The following detailed description is directed to apparatus and methodsfor providing a boundary layer flow sensor and corresponding use fordetermining the flow characteristics of the ambient airflow over thesensor. Specifically, the boundary layer flow sensor described hereinmay be used to determine laminar or turbulent flow conditions associatedwith an aerodynamic surface. As discussed briefly above, it is oftenuseful to determining whether an airflow is laminar or turbulent over aparticular aerodynamic surface or at a particular location on thesurface. This determination may be useful in testing situations, such asduring wind-tunnel or flight testing of a particular aircraft component,or during actual flight operations of an aircraft.

Conventional techniques for determining laminar or turbulent flowinclude using an infrared camera to measure surface temperature, using ahot film to measure sheer stress, or using a pressure tube mounted onthe surface to measure the total pressure of the airflow. With respectto the infrared camera technique, the difference between heat transferproperties of laminar and turbulent airflows, a temperature gradient atthe transition location may be seen with an infrared camera. However,this technique is not practical in many situations, such as use duringflight operations or to determine the airflow characteristics at alocation that is not readily visible.

Hot film may be embedded in a layer on the aerodynamic surface and usedto measure sheer stress based on the heat transfer in the location ofthe film. The fluctuation content associated with that measurement maybe used to determine the characteristics of the airflow. However, thehot film is not very durable, requiring special care. As a result, thismethod for determining the airflow characteristics is not ideally suitedfor flight operations.

Pressure tubes mounted to an aerodynamic surface provide an accuratemeasure of the total pressure of the airflow directed into the tubes.This pressure may be used to determine a velocity gradient of theairflow within the boundary layer. Because the velocity gradient at thesurface associated with a turbulent boundary layer is characteristicallyhigher than the velocity gradient of the same airflow in a laminarboundary layer, the state of the airflow may be determined as beinglaminar or turbulent at the location of the pressure tube. A problemwith these types of conventional measurements using pressure tubes isthat the tubes are mounted externally to the aerodynamic surface, whichcreates drag and is not practical for flight operations. Additionally,forward facing pressure tubes may get contaminated by foreign objectssuch as grit, dirt, ice, water, or other particles in the environmentand ambient airflow, or induced by the environment due to changes intemperature or pressure.

Utilizing the concepts and technologies described herein, a boundarylayer flow sensor may be mounted within an aerodynamic surface andtransitioned between open and closed states. In the open state, apressure port is exposed to a lower portion of the boundary layer of anambient airflow in order to measure total pressure, as well as staticpressure according to some embodiments. In the closed state, thepressure port is decoupled from the ambient airflow such that the sensoris substantially flush with the aerodynamic surface to allow for laminarairflow and to prevent contamination of the pressure port.

References are made to the accompanying drawings that form a parthereof, and which are shown by way of illustration, specificembodiments, or examples. Like numerals represent like elements throughthe several figures. Turning now to FIG. 1, a boundary layer flow sensor102 is shown according to one embodiment. The boundary layer flow sensor102 is shown according to two operational states, a closed state 112 andan open state 116. The boundary layer flow sensor 102 may be selectivelyre-configured between the closed state 112 and the open state 116 via anactivation mechanism 114 during flight operations or testing of anaircraft in which the boundary layer flow sensor 102 is mounted. Theactivation mechanism will be described in detail below with respect toFIG. 8. By providing a closed state 112 and an open state 116, theboundary layer flow sensor 102 may be effectively utilized to measurepressures within the ambient airflow over an aerodynamic surface 104when desired, while protecting the boundary layer flow sensor 102 fromcontaminants and preventing disruption of the ambient airflow when notused to acquire pressure measurements.

According to the embodiment shown in FIG. 1, the boundary layer flowsensor 102 includes a body 106 mounted within or below the aerodynamicsurface 104. The aerodynamic surface 104 may be an aircraft wing, flightcontrol surface, stabilizer, fuselage or any surface over which anambient airflow moves and in which it is desirable to determine whetherthat airflow is laminar or turbulent. The body 106 is mounted in aposition such that the boundary layer flow sensor 102 is substantiallyflush with the aerodynamic surface 104. While this embodiment shows atop surface of the boundary layer flow sensor 102 that is substantiallycoplanar with the aerodynamic surface 104, other embodiments describedherein may include a lid or other component that slightly projects abovethe aerodynamic surface 104. Accordingly, for the purposes of thisdisclosure, “substantially flush” may include coplanar arrangements aswell as projections above the aerodynamic surface 104. To create acoplanar arrangement, the top surface of the boundary layer flow sensor102, the body 106, the lid or moveable cover of the boundary layer flowsensor 102, the fastener in which a moveable shaft of the boundary layerflow sensor 102 is positioned, or any combination of these components,may be countersunk within the aerodynamic surface 104.

As will be described in further detail below, the body 106 may be formedfrom a fastener that is typically used on the aerodynamic surface 104that has been hollowed to accommodate the components of the boundarylayer flow sensor 102. Alternatively, the body 106 may be sizedaccording to a typical fastener so that the boundary layer flow sensor102 may be mounted within a fastener aperture that already exists in theaerodynamic surface 104. In other embodiments, apertures may beincorporated into any portion of the aerodynamic surface 104 in which itwould be desirable to have a boundary layer flow sensor 102 mountedwithin.

The boundary layer flow sensor 102 of FIG. 1 has a pressure tube 110extending along a longitudinal axis of the body 106. The pressure tube110 provides a pathway for the ambient airflow captured at the pressureport 118 to a pressure transducer or to an associated tube leading topressure transducer, as will be described in greater detail below withrespect to FIG. 5B. The pressure port 118 is created when the activationmechanism triggers the boundary layer flow sensor 102 to transition fromthe closed state 112 to the open state 116. The open state 116 fluidlycouples the pressure tube 110 to a lower portion of a boundary layer ofan ambient airflow over the aerodynamic surface 104, and the closedstate 112 decouples the pressure tube 110 from the boundary layer. Forthe purposes of this disclosure, the pressure port 118 may be consideredthe opening that couples the pressure tube 110 to the ambient airflow,allowing the associated air pressure to be measured.

Throughout the various embodiments described herein, the boundary layerflow sensor 102 is re-configurable between the closed state 112 and theopen state 116 to selectively protect and expose the pressure port 118.However, various embodiments will be described to provide variousmechanisms for providing the closed state 112 and the open state 116,along with corresponding variations in the configuration of the pressureport 118. According to the embodiment shown in FIG. 1, the boundarylayer flow sensor 102 includes a body 106 that is fixed within or belowthe aerodynamic surface 104, and a movable cover 108. The moveable cover108 in this example includes a hinged cover 118 that pivots from a rearedge, extending the front edge of the hinged cover 118 upwards to createan air scoop, or opening, which couples the pressure tube 110 to theambient airflow to create the pressure port 118.

Turning now to FIG. 2, an alternative embodiment of a boundary layerflow sensor 102 having a moveable cover 108 will be described. Accordingto this embodiment, the boundary layer flow sensor 102 is similar to thesensor described with respect to FIG. 1, including the transitionbetween the closed state 112 and the open state 116 utilizing a moveablecover 108. However, according to this implementation, the moveable cover108 includes a deformable cover 218. The deformable cover bucklesoutward or other wise deforms from a substantially planar configurationrepresented by the closed state 112 to a curved, bent, or otherwisebuckled configuration shown with respect to the open state 116. As willbe described in greater detail below, the activation mechanism 114 forproviding the transition between the closed state 112 and the open state116 may include the use of a shape memory allow within the deformablecover 218 that is configure to deform to create the open state 116according to a threshold temperature, or may include a metal or materialmanufactured with an internal bias or torque that may be “popped” to thedeformed configuration to create the open state 116 utilizing arelatively small mechanical force that may be triggered electrically orvia a diaphragm or other pressure or temperature sensitive mechanism.

FIGS. 3A and 3B show top perspective and side views, respectively, of aboundary layer flow sensor 102 in a closed state 112 and an open state116 according to another embodiment. In this embodiment, the boundarylayer flow sensor 102 includes a movable shaft 302 positioned within thebody 106. The activation mechanism 114 may be responsive to an ambientcondition, such as temperature or pressure, or to a control input orpredetermined condition or action to transition the boundary layer flowsensor 102 between the closed state 112 and the open state 116. In doingso, the movable shaft 302 is raised from within or below the aerodynamicsurface 104 to create the pressure port 118 by exposing the pressuretube 110 to the ambient airflow. It should be appreciated that the body106 of this embodiment may include a fastener that has been hollowed outor otherwise configured to accept the moveable shaft 302. Consequently,the moveable shaft 302 may be sized according to a particular fastenerin which it is to be positioned.

FIGS. 4A and 4B show a similar embodiment in which the top portion 402of the moveable shaft 302 is substantially conically shaped tocompliment and nest within a conically shaped recess 404 of the body 106when configured in the closed state 112. In doing so, the pressure tube110 is better sealed when closed, providing better protection.

FIGS. 5A and 5B show top perspective and side views, respectively, of analternative embodiment of a boundary layer flow sensor 102 in a closedstate 112 and an open state 116. According to this embodiment, theboundary layer flow sensor 102 has a moveable shaft 302 that raises andlowers to selectively expose or close the pressure port 118. Thisembodiment is similar to that described above with respect to FIGS. 3Aand 3B, however, as seen in FIGS. 5A and 5B, the boundary layer flowsensor 102 of this example includes a lid 502 attached to the movableshaft 302. As shown, when the movable shaft 302 is lowered to the closedstate 112, the lid 502 may be substantially flush with the aerodynamicsurface 104. The thickness of the lid 502 may be sufficiently thin as toallow for laminar airflow over the lid 502 and the aerodynamic surface104 without triggering a transition to turbulent airflow. For someimplementations, this thickness may be less than 0.01 inches.

An advantage provided by the lid 502 is that it may provide additionalprotection for the moveable shaft 302 from contamination entering thespace around the moveable shaft 302 within the body 106. In particular,one embodiment utilizes an o-ring 504 to provide a watertight sealbetween the lid 502 and the top surface of the boundary layer flowsensor 102 when configured in the closed state 112. It should beappreciated that while not shown in the figures, an o-ring 504 or otherseal may be utilized with any embodiment utilizing a moveable shaft 302.The o-ring 504 may be fitted into the body 106 surrounding the moveableshaft 302. It should also be appreciated that while not shown in thefigures, a shallow recess in the body 106 may be utilized to furtherenhance the seal when closed. This may also assist in providing asufficiently thin lid for preserving laminar flow.

FIGS. 5A and 5B also illustrate two different embodiments for convertingthe air pressure within the pressure port 118 to a pressure measurementto be used in the determination of the ambient airflow characteristics.To convert the air pressure to a value, a pressure transducer isconventionally used. FIG. 5A shows a local transducer 506 that iscoupled to the pressure tube 110 via a direct connection with the body106 or moveable shaft 302. FIG. 5B shows an external transducer 508 thatis coupled to the pressure tube 110 via a transducer tube 510. Thetransducer tube 510 is simply a tube that fluidly connects the pressuretube 110 to the external transducer 508 located in a separate locationfrom the boundary layer flow sensor 102. The local transducer 506 mayprovide a more compact solution as only electrical signals aretransported away from the boundary layer flow sensor 102. The externaltransducer 508 may include a number of transducer tubes 510 from anumber of boundary layer flow sensors 102 to feed into one or moremulti-channel transducers located at a central location. The disclosureherein is not limited to any particular type of pressure transducer.

FIG. 6 shows a cross-sectional view of a boundary layer flow sensor 102according to one embodiment. As seen in this example, the boundary layerflow sensor 102 includes a moveable shaft 302 with a lid 502. Thisexample illustrates a securing mechanism 606 positioned on the inside ofthe aircraft wing or other component housing the boundary layer flowsensor 102, on an opposite side of the aerodynamic surface 104, and isused to secure the body 106 in place. The securing mechanism 606 may bethreaded and in conjunction with the body 106, may operate like a nutand bolt to hold the boundary layer flow sensor 102 in place within orbelow the aerodynamic surface. Alternatively, any type of securingmechanism may be used, including but not limited to inducing deformationin the body 106 during installation to hold the body 106 in place(similar to a rivet), tabs on the body 106 that engage the aerodynamicsurface 104, or the like.

One aspect of the boundary layer flow sensor 102 shown in FIG. 6 thatvaries from the embodiments described above is that the boundary layerflow sensor 102 in this example is configured to not only measure thetotal pressure, but also the static pressure within the lower portion ofthe boundary layer of the ambient airflow. The pressure tube 110 in theembodiments described above with respect to FIGS. 1-5 is configured tomeasure total pressure of the ambient airflow within the lower portionof the boundary layer. In this example, there are multiple pressuretubes 110. In particular, the pressure tubes 110 include a totalpressure tube 602 configured to measure the total pressure of theambient airflow within the lower portion of the boundary layer and atleast one static pressure tube 604 configured to measure the staticpressure within the lower portion of the boundary layer. Like the totalpressure tube 602, each static pressure tube 604 is positioned along thelongitudinal axis of the body 106 such that when the pressure port 118is configured in the open state 116, the static pressure tube 604 isfluidly coupled to the lower portion of the boundary layer.

According to one embodiment, the boundary layer flow sensor 102 includesa total pressure tube 602 and two static pressure tubes 604 positionedat offset angles 702 of approximately 60 degrees in both directions fromthe total pressure tube 602. While the example shown in FIG. 6 mayinclude two static pressure tubes 604, the second static pressure tube604 is not visible behind the total pressure tube 602 according to thatview. To better visualize the configuration having a total pressure tube602 and two static pressure tubes 604, FIG. 7 shows a top perspectiveview of the top portion of the boundary layer flow sensor 102 of FIG. 6.The boundary layer flow sensor 102 is rotated in the view shown in FIG.7 so that the total pressure tube 602 is facing directly outward intothe ambient airflow, which would be flowing into the page.

The boundary layer flow sensor 102 has two static pressure tubes 604positioned at offset angles 702 from either side of the total pressuretube 602. The offset angles 702 may be approximately 60 degrees asmeasured from the position of the total pressure tube 602. According toother embodiments, the offset angles 702 may include angles within therange of 40-70 degrees from the position of the total pressure tube 602.It should be appreciated that the disclosure herein contemplates anyoffset angle 702 that provides for a static pressure measurement to betaken, without being limited to any particular offset angle value. Thetwo static pressure tubes 604 feed into the same chamber (not shown) andallow for an average to be taken to improve accuracy of the staticpressure measurement. By utilizing two static pressure tubes 604 toobtain an average, a small miss-alignment of the boundary layer flowsensor 102 with respect to the local flow direction resulting in anincrease on pressure on one side will result in a decrease in pressureon the other side. Thus, the average provides a good measure of thestatic pressure even with a miss-alignment.

FIG. 8 is a block diagram illustrating various embodiments of anactivation mechanism 114 according to the disclosure provided above. Aspreviously discussed, the activation mechanism 114 provides thetransition between the closed state 112 and the open state 116 of theboundary layer flow sensor 102. Each of the embodiments described abovewith respect to FIGS. 1-7 may use one or more of various types ofactivation mechanisms 114. FIG. 8 shows various contemplated activationmechanisms 114 presented according to the type of trigger 802 to whichthe activation mechanism 114 is responsive to transition the boundarylayer flow sensor 102 between the closed state 112 and the open state116. The triggers 802 may be generally considered as being an ambientcondition or an electrical or other type of selectable or preprogrammedswitch.

An ambient condition may include a temperature threshold or a pressurethreshold. An example of an activation mechanism 114 responsive to atemperature threshold is a shape memory alloy 804. Shape memory alloys804 predictably change shape in response to being exposed to aparticular temperature threshold. After transitioning back through thattemperature threshold, the shape memory alloys 804 will return to theiroriginal shape. In the context of this disclosure, shape memory alloys804 may be utilized in at least two ways. First, the deformable cover218 described above with respect to FIG. 2 may be manufactured from ashape memory alloy 804 that is shaped to provide the closed state 112 attemperatures above a predetermined threshold, and in response to theambient temperature dropping below the threshold temperature, the shapememory alloy 804 used in the deformable cover 218 will change shape tocreate the open state 116 shown and described above.

Another manner in which a shape memory alloy 804 may be used in thecontext of this disclosure is to be directly or indirectly attached tothe moveable shaft 302. In response to the ambient temperaturedecreasing beyond a threshold temperature, the shape memory alloy 804deforms to apply a force or pressure against the moveable shaft 302 toraise the shaft and create the pressure port 118 in the open state 116.An increase in temperature beyond the threshold temperature would returnthe shape memory alloy to its original shape, which would in turn lowerthe moveable shaft 302, closing the pressure port 118 and creating theclosed state 112. This concept may also be applied to providing themovement of the hinged cover 118.

The activation mechanism 114 may also be responsive to a pressurethreshold as a trigger 802 to open or close the boundary layer flowsensor 102. An example implementation includes utilization of adiaphragm 806 that expands and contracts in response to a decrease ofambient air pressure as altitude increases, and a corresponding increaseof ambient air pressure as altitude decreases. The expansion andcontraction of the diaphragm 806 may be used to physically push and pullthe moveable shaft 302, as well as the hinged cover 118, to transitionthe pressure port 118 between the closed state 112 and the open state116.

As mentioned above, in addition to an ambient condition, the trigger 802for the activation mechanism 114 may include an electrical or other typeof selectable or preprogrammed switch. As an example, a pilot or othercrew member or engineer may press a physical or virtual button toselectively activate and deactivate the boundary layer flow sensor 102to transition the sensor to the open state 116 and back to the closedstate 112, respectively. Similarly, an aircraft system and associatedcontroller may be programmed to activate and deactivate the boundarylayer flow sensor 102 when a predetermined condition is satisfied. Thepredetermined condition may depend on the location of the particularboundary layer flow sensor 102 and on the timing and/or positionassociated with a desired laminar or turbulent airflow determination. Asan example, the activation mechanism 114 may be programmed to create theopen state 116 at a particular altitude, attitude, angle of attack,velocity, engine setting, flight control setting, stage of flight, orambient condition such as temperature or pressure.

A trigger 802 that is electrical or other may include a number of typesof actuators 808. Examples include, but are not limited to,electro-mechanical actuators 810, piezo-electric actuators 812,solenoids 814, and any others 816. It should be understood that whilemultiple activation mechanisms 114 having a trigger 802 that iselectrical, any other type of trigger 802 may also be used withoutdeparting from the scope of this disclosure. Examples of other types ofselectable or preprogrammed triggers and corresponding actuators 808 mayinclude, but are not limited to, hydraulic, pneumatic, and magnetic.

Turning now to FIG. 9, an illustrative routine 900 for determining theflow characteristics of an ambient airflow over an aerodynamic surface104 will now be described in detail. It should be appreciated that moreor fewer operations may be performed than shown in FIG. 9 and describedherein. Moreover, these operations may also be performed in a differentorder than those described herein. The routine 900 begins at operation902, where a boundary layer flow sensor 102 that is mountedsubstantially flush with the aerodynamic surface 104 is activated. Asdescribed in detail above with respect to FIG. 8, the activationmechanism 114 for activating the boundary layer flow sensor 102 may betriggered in various ways, including in response to ambient conditionsor in response to an electrical or other selectable or preprogrammedswitch.

From operation 902, the routine 900 continues to operation 904, where inresponse to activating the boundary layer flow sensor 102, theactivation mechanism 114 exposes the pressure port 118 within the lowerportion of the boundary layer of the ambient airflow. This operationrepresents the transition from the closed state 112 to the open state116, which may include the opening of the moveable cover 108 or theraising of the moveable shaft 302. The routine 900 continues fromoperation 904 to operation 906, where a pressure measurement is takenwithin the lower portion of the boundary layer. This pressuremeasurement is taken with the pressure tube 110, including at least atotal pressure tube 602, and according to one embodiment, two staticpressure tubes 604 positioned at offset angles 702 of approximately 60degrees in both directions from the total pressure tube 602. Atoperation 908, a determination is made from the pressure measurementwhether the ambient airflow is substantially laminar or turbulent, andthe routine 900 ends.

The determination of whether the airflow is laminar or turbulent isbased on the characteristics of the lower portion of the boundary layerof the ambient airflow where the pressure port 118 is exposed. FIG. 10shows a graph 1000 illustrating a pressure parameter, p*, versusdistance along a chord, x/c, for both laminar and turbulent flows. Thedistance along the chord, x/c, represents locations along theaerodynamic surface 104 from a position upstream of the boundary layerflow sensor 102 to a position downstream of the boundary layer flowsensor 102. The position of the boundary layer flow sensor 102,represented by the vertical line labeled “sensor location,” may be anyposition of interest in which a determination of laminar or turbulentflow is desired.

The pressure parameter, p*, represents a variable that correlates to thedifference between total pressure and static pressure, normalized by afree-stream reference dynamic pressure. According to this example,p*=(P _(t) −P _(s))/q, with q=(½ρU _(inf) ²), ρ=[ambient air density],and U _(inf)=[free air velocity]

Looking at the graph 1000, the solid line representing the laminar plot1002 corresponds to the plot of the pressure parameter of a laminarairflow within the lower portion of a boundary layer from a position x/cupstream of the boundary layer flow sensor 102 to a position x/cdownstream of the boundary layer flow sensor 102. Similarly, the brokenline representing the turbulent plot 1004 corresponds to the plot of thepressure parameter of a turbulent airflow within the lower portion ofthe boundary layer from a position x/c upstream of the boundary layerflow sensor 102 to a position x/c downstream of the boundary layer flowsensor 102. As mentioned above, the velocity gradient at the surface fora turbulent flow is higher than that of a corresponding laminar flow.Correspondingly, the pressure parameter, p*, is higher for turbulentflow as compared to laminar flow. The transition from laminar flow toturbulent flow is identified on the graph 1000 with a “transition” labeland arrow pointing to the steep increase in the turbulent plot 1004upstream of the boundary layer flow sensor 102. At the position of theboundary layer flow sensor 102, represented by the vertical line andlabel, the pressure parameter associated with a turbulent flow isidentified with a turbulent pressure parameter 1006, while the pressureparameter associated with a laminar flow is identified with a laminarpressure parameter 1008.

When a determination of the flow characteristics, laminar versusturbulent flow, of an ambient airflow over an aerodynamic surface isdesired, the boundary layer flow sensor 102 is activated to take one ormore pressure measurements within the lower portion of the boundarylayer of the airflow at the desired location. When the boundary layerflow sensor 102 includes both a total pressure tube 602 and staticpressure tubes 604, measurements of total pressure, P_(t), and of staticpressure, P_(s), may be taken and used to determine the correspondingpressure parameter, p*. If the boundary layer flow sensor 102 does notutilize a static pressure tube 604, then the static pressure may beestimated using a priori information.

To utilize the total pressure and static pressure to determine thestate, or characteristics, of the ambient airflow, several estimates anddeterminations may be made to arrive at the various components of thegraph 1000, and ultimately to the pressure parameter, p*. Having thepressure parameter then allows for the determination as to whether thatpressure parameter falls on or above the turbulent plot 1004 to indicateturbulent flow, on or below the laminar plot 1002 to indicate laminarflow, or within the transition area between the laminar plot 1002 andthe turbulent plot 1004 to indicate a boundary layer that istransitioning between laminar and turbulent flows.

First the boundary layer edge velocity is estimated utilizing acoefficient of pressure that is determined from computational fluiddynamics or measured by the boundary layer flow sensor 102. The precisealgorithms for these estimates will not be provided as they aredetermined using computational fluid dynamics or other known techniques.Next, the velocity at the boundary layer flow sensor 102 is estimatedfor laminar and turbulent flows according to the position and height ofthe sensor. The sizing and positioning of the sensor will be describedin greater detail below. Using the estimated velocities for laminar andturbulent flows at the position of the boundary layer flow sensor 102,the estimated values of the pressure parameter, p*, may be determinedfor both laminar and turbulent flows. After taking measurements with theboundary layer flow sensor 102 to determine total pressure and staticpressure (or estimating static pressure) the pressure parameter for thatmeasurement may be determined as discussed above, and compared with theestimated values of the pressure parameter per the graph 1000 todetermine whether to flow is laminar, turbulent, or transitional. Itshould be appreciated that the estimated pressure parameter values forlaminar and turbulent flow may be determined using actual flight test orwind tunnel test results. In other words, measurements may be taken andpressure parameters determined according to known laminar flowconditions, and then again after intentionally transitioning theboundary layer to turbulent flow conditions.

According to the various embodiments described herein, the boundarylayer flow sensor 102 exposes the pressure port 118 to the lower portionof the boundary layer of the ambient airflow. Because turbulent flow hasa substantially higher velocity gradient than laminar flow at the bottomof the boundary layer, the boundary layer flow sensor 102 is sized toexpose the pressure port 118 to this lower portion of the boundary layerwhen configured in the open state 116. In order to determine the stateor characteristics of the boundary layer ambient airflow using a singleposition measurement of by the boundary layer flow sensor 102, a prioriinformation is used to estimate the laminar boundary layer thickness atthe location of the boundary layer flow sensor 102 in order to size thesensor accordingly. Computational fluid dynamics or other knowntechniques may be used to calculate the laminar displacement thicknessof the boundary layer at the position of interest where the boundarylayer flow sensor 102 is to be positioned.

The height of the boundary layer flow sensor 102, and specifically ofthe pressure port 118, may then be determined based on the lowestexpected displacement thickness of the laminar boundary layer at theposition of the sensor. The height of the pressure port 118 may then besized to be less than the lowest expected displacement thickness.According to one example, the height is sized at 75% of the lowestexpected displacement thickness of the laminar boundary layer at theposition of the boundary layer flow sensor 102. The height of thepressure port may be considered the distance from the aerodynamicsurface 104 to the underside of the lid 502, or from the aerodynamicsurface 104 to the top of the opening hole of the pressure port 118 ifthere is no lid 502 on the boundary layer flow sensor 102.

It should be clear from the disclosure above that the technologiesdescribed herein provide for a boundary layer flow sensor 102 that iscapable of providing a single point measurement of pressure within alower portion of a boundary layer to accurately determine whether thecorresponding airflow is laminar or turbulent. According to the variousembodiments, the boundary layer flow sensor 102 provides an open state112 and a closed state 116 that allows for selective activation of theboundary layer flow sensor 102 for pressure measurements, whileprotecting the boundary layer flow sensor 102 from contamination whennot in use.

The subject matter described above is provided by way of illustrationonly and should not be construed as limiting. Various modifications andchanges may be made to the subject matter described herein withoutfollowing the example embodiments and applications illustrated anddescribed, and without departing from the true spirit and scope of thepresent disclosure, which is set forth in the following claims.

What is claimed is:
 1. A boundary layer flow sensor, comprising: a bodyconfigured for mounting below an aerodynamic surface and having apressure tube extending along a longitudinal axis of the body; apressure port configurable between an open state and a closed state, thepressure tube fluidly coupled to a lower portion of a boundary layer ofan ambient airflow over the aerodynamic surface when the pressure portis in the open state, and the pressure tube fluidly decoupled from theboundary layer when the pressure port is in the closed state; and anactivation mechanism operative to fluidly couple and decouple thepressure tube to the boundary layer to provide the pressure port in theopen state and closed state, respectively.
 2. The boundary layer flowsensor of claim 1, wherein the activation mechanism comprises a movablecover separating the pressure tube from the ambient airflow, the movablecover movable between a lowered position in which the pressure port isin the closed state and a raised position in which the pressure port isin the open state.
 3. The boundary layer flow sensor of claim 2, whereinthe movable cover comprises a hinged cover configured to pivot along adownstream edge.
 4. The boundary layer flow sensor of claim 2, whereinthe movable cover comprises a deformable cover operative to change shapeto reconfigure the pressure port between the open state and the closedstate.
 5. The boundary layer flow sensor of claim 4, wherein thedeformable cover comprises a shape memory alloy configured to changeshape in response to a threshold temperature.
 6. The boundary layer flowsensor of claim 4, wherein the deformable cover comprises a materialhaving an internal bias configured to switch between the open state andthe closed state.
 7. The boundary layer flow sensor of claim 1, furthercomprising a moveable shaft positioned within the body and comprisingthe pressure tube, and wherein the activation mechanism is responsive toan ambient condition to raise the movable shaft to configure thepressure port in the open state and to lower the movable shaft toconfigure the pressure port in the closed state.
 8. The boundary layerflow sensor of claim 7, wherein the activation mechanism comprises ashape memory alloy, and wherein the ambient condition comprises atemperature threshold.
 9. The boundary layer flow sensor of claim 7,wherein the activation mechanism comprises a diaphragm, and wherein theambient condition comprises a pressure threshold.
 10. The boundary layerflow sensor of claim 7, wherein the activation mechanism comprises apiezo-electric actuator or a solenoid.
 11. The boundary layer flowsensor of claim 7, further comprising a lid attached to the movableshaft such that when the movable shaft is lowered, the lid is flush withthe aerodynamic surface.
 12. The boundary layer flow sensor of claim 7,further comprising a lid attached to the movable shaft such that whenthe movable shaft is lowered, the lid protrudes above the aerodynamicsurface, wherein the lid comprises a thickness that allows for laminarairflow over the lid and the aerodynamic surface.
 13. The boundary layerflow sensor of claim 7, wherein the body comprises a fastener andwherein the moveable shaft is sized according to the fastener.
 14. Theboundary layer flow sensor of claim 1, further comprising a pressuretransducer fluidly coupled to the pressure tube and operative to producean electrical signal corresponding to the total pressure associated withthe ambient airflow within the lower portion of the boundary layer. 15.The boundary layer flow sensor of claim 1, wherein the pressure tubecomprises a total pressure tube configured to measure a total pressureof the ambient airflow within the lower portion of the boundary layer,and wherein the boundary layer flow sensor further comprises at leastone static pressure tube positioned along the longitudinal axis of thebody such that when the pressure port is configured in the open state,the at least one static pressure tube is fluidly coupled to the lowerportion of the boundary layer at an offset angle from the total pressuretube.
 16. The boundary layer flow sensor of claim 15, the at least onestatic pressure tube comprises two static pressure tubes positioned atoffset angles of approximately 60 degrees in both directions from thetotal pressure tube.
 17. A boundary layer flow sensor, comprising: abody configured for mounting within a fastener aperture in anaerodynamic surface, the body comprising a movable shaft that ismoveable between a raised position to create an open state for apressure port and a lowered position flush with the aerodynamic surfaceto create a closed state for the pressure port, the moveable shafthaving a total pressure tube extending along a longitudinal axis of themovable shaft and aligned with an ambient airflow when the pressure portis in the open state, and at least one static pressure tube positionedat an offset angle from the total pressure tube; and an activationmechanism operative to raise the movable shaft and fluidly couple thetotal pressure tube and the at least one static pressure tube to a lowerportion of a boundary layer of the ambient airflow, and to lower themovable shaft to the lowered position, in which the moveable shaft isflush with the aerodynamic surface, and fluidly decouple the totalpressure tube and the at least one static pressure tube from the lowerportion of the boundary layer of the ambient airflow.
 18. A method fordetermining flow characteristics of an ambient airflow over anaerodynamic surface, the method comprising: activating a boundary layerflow sensor mounted flush with the aerodynamic surface; in response toactivating the boundary layer flow sensor, exposing a pressure port to alower portion of a boundary layer of the ambient airflow; measuring atleast one pressure associated with the ambient airflow within the lowerportion of the boundary layer; and determining according to the at leastone pressure, whether the ambient airflow is laminar or turbulent. 19.The method of claim 18, wherein exposing the pressure port to the lowerportion of the boundary layer of the ambient airflow comprises moving amoveable cover attached to a top end of a body of the boundary layerflow sensor to fluidly couple a pressure tube within the body to theboundary layer of the ambient airflow.
 20. The method of claim 18,wherein exposing the pressure port to the lower portion of the boundarylayer of the ambient airflow comprises raising a moveable shaft with apressure tube from below the aerodynamic surface to fluidly couple thepressure tube to the boundary layer of the ambient airflow.